The use of data transmission in wireless networks is increasing rapidly. One effect of new transmission technologies and operator business models is that data transmission capacity can be seen as a commodity. The increase in consumption is not only related to increasing bandwidth requirements but also verbose application design and the large number of users. Each generation of telecommunication technology has enabled higher data rates and lower delays which are important for the user experience. With the next emerging generation of telecommunication technology, Long Term Evolution (LTE), new challenges aiming at even more efficient use of radio resources have to be overcome in order to satisfy the demand for efficient data transmissions.
The Uplink (UL) transmission in a wireless LTE network relates to transmission of user data from a User Equipment (UE) to a Base Station (BS) of a wireless communication cell. The Base Station may commonly be referred to as an evolved Node B (eNB). The UL transmission in an LTE enabled cell is typically administered by dividing the UL into a plurality of Resource Blocks (RBs). As illustrated in FIG. 1, two resource blocks (RBs) defines in the time domain one sub-frame, having a typical duration of 1 ms, whereas in the frequency domain one resource block typically comprise 12 sub-carriers. The technology used for UL transmission, in LTE, is Single Carrier FDMA. Consequently, if two RBs are assigned to a UE in the same subframe they must be consecutive in the frequency domain. The resource block of FIG. 1 may also be used for downlink transmission where the transmissions to a UE may be distributed in frequency, using frequency diversity schemes as is know in the art.
FIG. 5 is a signaling scenario illustrating a typical procedure for a UE requiring resources for data transmission. UE 100 having data to send requests UL resources by transmitting a Scheduling Request (SR) to an eNB 110, as indicated with a first step 1:1. The eNB 110 responds to such a request by performing resource scheduling, as indicated with a next step 1:2, wherein it assigns UL resources, i.e. one or several RBs, to UE 100, and decides on an appropriate UL transport format for the assigned UL resources. The transport format could comprise parameters defining e.g. Transport Block (TB) size, physical layer coding and multiplexing. Thereby, eNB 110 will be aware of the UL transmission format to be applied for the data transmission before the data is received. The parameters specified for the allocated radio resources are issued by eNB 110 and transmitted to UE 100 in a Schedule Grant (SG), as indicated with a subsequent step 1:3. By analyzing the received SG, UE 100 can find out the transport format and the RBs it has been allocated by eNB 110 and, on the basis of this information UE 100 can initiate the requested UL transmission to eNB 110, as indicated with a final step 1:4.
Scheduling of UL resources to UEs is a complex problem where numerous factors and parameters may have to be accounted for. One parameter to take into consideration could be the amount of data, presently stored in its respective send buffer that the UE has to send. The UE sends Buffer Status Reports (BSR) to the scheduler in the eNB, which uses these reports to prioritize and allocate resources between different UEs. A BSR may typically comprise information regarding the buffer sizes for corresponding logical channels.
FIG. 8 shows another signaling scheme, describing in general manner, the steps to be taken when data becomes available for transmission at a UE 100 and if this data belongs to a radio bearer (logical channel) group having a higher priority than those for which data already exists in the buffer, or if the UE 100 buffer was empty just before this new data became available for transmission, as indicated with a step 1:1. The UE 100 transmits a SR to the eNB in order to acquire UL resources, as indicated with another step 1:2. In response to the request of step 1:2, eNB 130 schedules RBs, as indicated with another step 1:3, and responds by transmitting a SG to the UE 100, as indicated with a subsequent step 1:4, after which UE 100 may initiate data and BSR transmissions in the UL, as indicated in a final step 1:5.
With this type of triggering mechanism, the eNB 130 can quickly be made aware when data with higher priority is available for transmission at UE 100, without having to request any excessive reporting from UE 100. The trigger of a regular BSR also triggers an SR. Another type of BSR, typically referred to as the periodic BSR, provides a timer-based trigger per UE, enabling reporting for continuous flows.
Different Transport Block (TB) sizes are available for data transmission. In case a TB size is larger than the amount of data available for transmission at the time of assembly of the Media Access Control (MAC) Protocol Data Unit (PDU) to be used for the transmission, one BSR, commonly referred to as a padding BSR, can also be included.
In order to increase throughput in a wireless cell, certain transmission schemes are currently employed. One commonly used scheme suitable for increasing the data rate between a UE and the eNB is Single User Multiple Input Multiple Output (SU-MIMO), which is a scheme utilizing at least two concurrent data streams for data transmission between a UE and the eNB. Applying this scheme increases the data rate between the UE and the eNB. However, this scheme relies on the fact that the send buffer of the UE is sufficiently large to bring satisfying gains.
LTE supports Multiple User (MU)-MIMO in the uplink (UL) and the downlink (DL). MU-MIMO schemes are configured to support multiple UEs sending simultaneously to an eNB by utilizing spatial multiplexing of the UEs. Even though this will cause intra-cell interference, the interference can be limited if the UEs are carefully selected. By defining a sufficiently high channel quality, commonly referred to as Signal-to-Interference-plus-Noise-Ratio (SINR) for a UE, there is a possibility to identify UEs feasible to be involved in MU-MIMO. The selection of two, or in some cases, even more UEs for which MU-MIMO is to be applied may further be based on various factors such as spatial correlation of reference signals or sounding reference signals received from different UEs to acquire information of channel quality and spatial position of the UEs. In the uplink, the eNB measures the UL channel quality through sounding and demodulation reference symbols and thereby estimates the SINR.
Hence, MU-MIMO performance is generally affected by parameters such as e.g. SINR distribution, scheduling priorities and channel estimation. Current MU-MIMO schemes are designed to optimize with a focus on the most demanding UEs, i.e. UEs with high data rate services. Further, in order not to diminish each UE's level of data per RB, two UEs with high SINR are preferred.
Consequently, in order to increase the cell's efficiency by employing MU-MIMO, many UEs with high respective SINRs are required to be able to identify a number of UEs which are suitable for co-sending on the same RBs. If the respective SINR is not high enough, utilization of MU-MIMO may be less efficient than SU-MIMO for the UEs since the UEs amount of data per RB usually decreases. This implies that the highest throughput gains are accomplished when UEs with both high buffer data rates and high SINR are employed in MU-MIMO.
Coordinated Multi-Point (CoMP) transmission and reception refers to a system where the transmission and/or reception at multiple, geographically separated antenna sites are dynamically coordinated in order to improve system performance. The coordination can either be distributed, by means of direct communication between the different sites, or centralized in a central coordinating node. In some scenarios, the antenna sites may be Radio Remote Units controlled by one single radio network node, such as an eNB.
CoMP is considered for International Mobile Telecommunications Advanced (IMT-Advanced) as a potential technique to improve the coverage for high data rate users, to improve the cell-edge throughput and/or to increase system throughput. In particular, the goal is to distribute the user perceived performance more evenly in the network by taking control of the inter-cell interference.
Downlink coordinated multi-point transmission implies dynamic coordination among multiple geographically separated transmission points, and may be categorized as follows. Coordinated scheduling and/or beamforming may be applied in situations where data to a single UE is instantaneously transmitted from one of the transmission points, and scheduling decisions are coordinated to control, e.g. the interference generated in a set of coordinated cells. Joint processing/transmission may be applied in situations where data to a single UE is simultaneously transmitted from multiple transmission points, e.g. to improve the received signal quality and/or actively cancel interference for other UEs. Moreover, uplink coordinated multi-point reception implies joint reception and processing of signals at multiple, geographically separated points. Scheduling decisions may be coordinated among cells to control interference.
In systems employing CoMP in the uplink, similar dynamic coordination among multiple transmission points as described above is required. For example, such coordination may be performed by exchange of interference covariance matrices.
Most CoMP technologies try to optimize the system settings for a full buffer scenario where all users can utilize their signal quality, such as SINR. In other scenarios, the buffers may not be full. Consider for example a typical low data rate service such as speech. When sent over an IP network it is called VoIP (Voice over IP). VoIP has a low data rate, but stringent delay requirements such that data must be sent periodically. For the AMR codec with a 12 kbps bit rate and 20 ms periodicity a typical VoIP packet is between 320-400 bits. When the user is listening, less information is sent in special packets called SID packets, which are approximately 120-150 bits. In addition to VoIP, gaming and certain M2M applications generate small packets. Furthermore, many background messages generated by PC applications are small and not bundled together. Thus, it is common to also send small packets, i.e. the buffers may not be full.
Therefore, a problem of prior art is that it is required that the UEs in the system require high data rates in order to fully make use of the schemes proposed for increased throughput.
US2009052371 discloses a method for distributed power control in a network. The method determines a transmit power for a plurality of transmitting nodes such that signals sent from each of the transmitting nodes are received at a receiving node at a signal to interference plus noise ratio (SINR) set point. The method includes increase and decrease of SINR such that an average SINR is maintained. An SINR for transmitting nodes having a data rate higher than an average data rate, is increased.